I. Field
The present invention relates generally to communication, and more specifically to techniques for controlling radiated power for a multi-antenna transmission.
II. Background
Wireless communication networks are widely deployed to provide various communication services such as data, voice, video, and so on. These networks include wireless wide area networks (WWANs) that provide communication coverage for large geographic areas (e.g., cities), wireless local area networks (WLANs) that provide communication coverage for medium-size geographic areas (e.g., buildings and campuses), and wireless personal area networks (WPANs) that provide communication coverage for small geographic areas (e.g., homes). A wireless network typically includes one or more access points (or base stations) that support communication for one or more user terminals (or wireless devices).
A wireless communication network typically operates in a designated frequency band. In the United States, the Federal Communications Commission (FCC) is a regulatory agency that restricts radiated power levels for various frequency bands to facilitate efficient use of these frequency bands and to avoid excessive RF interference. For example, an IEEE 802.11 WLAN typically operates in a U-NII band that covers 5.15 to 5.35 gigaHertz (GHz) and 5.725 to 5.825 GHz. Although the U-NII band is unlicensed, a wireless station (which may be an access point or a user terminal) needs to limit its radiated power in any spatial direction to within a level mandated by the FCC in order to qualify for operation in the U-NII band.
In general, it is desirable to use as much transmit power as possible in order to improve the signal-to-noise-and-interference ratio (SNR) for a data transmission and/or to extend the range of operation. A higher SNR can support a higher data rate and/or improve reliability for the data transmission. The radiated power in a given direction is determined by the amount of transmit power applied to the antenna(s) at a wireless station and an antenna pattern formed by the antenna(s). If the wireless station is equipped with multiple antennas, then these antennas may be used to synthesize an antenna pattern that increases the radiated power in a specific spatial direction, e.g., toward a receiving station. In general, the antenna pattern is often not easy to ascertain because it is dependent on various factors such as the physical attributes of each antenna, the layout and arrangement of the antennas, and so on. If the antenna pattern is not known, then the wireless station may assume a maximum possible gain for the antenna pattern and may set the transmit power level accordingly in order to meet regulatory limit. However, in many instances, the maximum gain is not achieved for any spatial direction, and setting the transmit power level based on this maximum gain results in a lower SNR and/or reduced range, both of which are undesirable.
There is therefore a need in the art for techniques to control radiated power for a multi-antenna transmission.